The cost per die of electronic components is reduced significantly as feature size becomes smaller. As device feature sizes become smaller, conventional lithographic processes become increasingly more difficult and expensive. Therefore, significant challenges are encountered in the fabrication of nanostructures, particularly structures having a feature size of less than a resolution limit of immersion photolithography (about 50 nm).
It is possible to fabricate isolated or semi-dense structures at this scale using a conventional lithographic process, such as shadow mask lithography and e-beam lithography. However, such processes are limited because the exposure tools are extremely expensive or extremely slow and, further, may not be amenable to formation of structures having dimensions of less than 50 nm.
The development of new lithographic processes, as well as materials useful in such processes, is of increasing importance in making fabrication of small-scale devices easier, less expensive, and more versatile. One example of a method of fabricating small-scale devices that addresses some of the drawbacks of conventional lithographic techniques is block copolymer lithography, where use is made of polymer masks derived from self-assembly of block copolymers. Block copolymers are known to form nano-scale microdomains by microphase segregation. In the fabrication of the block copolymer, the microdomains may rearrange into a self-assembled array by microphase segregation to achieve a thermodynamic equilibrium state by, for example, treating the block copolymer with at least one of heat and a solvent. When cast on a substrate and treated, block copolymers form nano-scale periodic patterns that may be useful as an etch mask in semiconductor device fabrication. Such ordered patterns of isolated structural units formed by the self-assembled block copolymers may potentially be used for fabricating periodic structural units and, therefore, have promising applications in, for example, semiconductor, optical, and magnetic devices. Dimensions of the structural units so formed are typically in the range of 5 nm to 50 nm, which dimensions are extremely difficult to define using conventional lithographic techniques. The size and shape of these domains may be dominated by controlling the molecular weight and composition of the block copolymer. Additionally, the interfaces between these domains have widths on the order of 1 nm to 5 nm and may be controlled by changing the chemical composition of the blocks of the copolymers.
An important factor in determining microphase segregation behavior in block copolymers for self-assembled lithography is the Flory-Huggins interaction parameter (χ value), which indicates an energetic penalty of dissolving one block into the other block. Accordingly, the χ value of a block copolymer defines a tendency of the block copolymer to segregate into microdomains as a function of the block copolymer's weight, chain length, and/or degree of polymerization. Chi has the generic relationship of χ=a+b/T, where T is a processing temperature and wherein a and b are material-specific values dependent on the block polymers. The χ value, the degree of polymerization (i.e., a number of monomer repeats in a block copolymer chain) (N), and the composition (Φ) of a block copolymer dictate the phase behavior of the block copolymer. The χN value of a block copolymer has ramifications on both the kinetics of self-assembly and the thermodynamic equilibrium state of the block copolymer. Microphase segregation occurs above a certain value of χN, where N is the number of monomer repeats in a block copolymer chain. If χN of a block copolymer is less than or equal to about 10, the disordered state has a lower free energy than the ordered state and the block copolymer domains do not phase separate. When χN is greater than about 10, the ordered state has lower free energy and the block copolymer domains phase separate into a variety of ordered periodic microstructures dependent on the volume fractions of each domain.
Materials with a greater χ value microphase segregate at a smaller chain length, yielding patterns with a smaller period. The width of an interface between the microdomains of the block polymer is given by αχ−1/2, where α is the statistical segment length, implying that a block copolymer with a greater χ value may have sharper and more distinct boundaries between the microdomains. Improved boundaries may result in a decrease in line edge roughness in features patterned using the segregated block copolymer. Since block copolymers having increased χ values between the polymer blocks thereof may provide arrays having reduced periodicity and increasingly smooth interfaces between microdomains, such block copolymers may enable formation of smaller features having reduced line edge roughness in semiconductor device fabrication. The accessible χ value of the block copolymer falls within a discrete range determined by a processing temperature range of the block copolymer (i.e., a temperature in a range of greater than or equal to a glass transition temperature (Tg) of the block copolymer and less than a decomposition temperature of the block copolymer).
However, a block copolymer having desired physical properties (i.e., one block inherently having etch selectivity over the other block) may not possess desirable processing qualities such as equilibrium defect density and annealing time (i.e., the time required to reach equilibrium self-assembled state), because of a limited range of accessible χ values. The mechanism for reaching equilibrium from a defect-ridden, as-cast state (i.e., defect reduction) requires movement of one block through the other so that the polymer chains may be positioned in lower energy configurations. The rate of this process decreases as the χ value increases and, thus, large χN values of a block copolymer may result in undesirable increases in the length of time for microphase segregation of the block copolymer to occur. Therefore, in block copolymers having large χN values, it is increasingly difficult to produce a self-assembled film having a tolerable equilibrium defect density within an acceptable time period for efficient fabrication of the semiconductor device.
For example, poly(styrene-b-dimethylsiloxane) block copolymer (PS-b-PDMS), exhibits desirable properties for use in lithographic techniques. PS-b-PDMS is a diblock polymer that includes polystyrene blocks and polydimethylsiloxane blocks. The chemical structure of PS-b-PDMS is shown in FIG. 1, where n represents a number of repeats of styrene in the polystyrene blocks and m represents a number of repeats of dimethylsiloxane in the polydimethylsiloxane blocks. PS-b-PDMS has a high χ value in comparison to other block copolymers that provides a high degree of phase separation as well as an etch-resistant PDMS block. Solvent vapor annealing has been used to speed up self-assembly time for the PS-b-PDMS. However, solvent vapor annealing may be difficult to perform and may results in an increase in defects specific to the solvent vapor annealing process in the ordered microdomains formed from the PS-b-PDMS.